1. Field of the Invention
The invention relates to a fuel cell separator (used for a solid polymer electrolyte type fuel cell or the like).
2. Description of the Related Art
A solid polymer electrolyte type fuel cell is composed of layered modules, each of which is composed of a membrane-electrode assembly (MEA) and a separator. The MEA is composed of an electrolytic membrane made of an ion-exchange membrane, an electrode (anode or fuel electrode) made of a catalytic layer disposed on one face of the electrolytic membrane, and an electrode (cathode or air electrode) made of a catalytic layer disposed on the other face of the electrolytic membrane. Diffusion layers are disposed between the anode-side catalytic layer and the separator and between the cathode side catalytic layer and the separator, respectively. A fuel gas flow channel through which a fuel gas (hydrogen) is supplied to the anode is defined by the separator on the side of the anode with respect to the MEA. An oxidative gas flow channel through which an oxidative gas (oxygen, air as a rule) is supplied to the cathode is defined by the separator on the side of the cathode with respect to the MEA. Terminals, insulators, and end plates are disposed at opposed ends of a layered-module body in a module-layering direction, whereby a stack is constructed. This stack is clamped in the module-layering direction and is fixed by means of bolts and a fastening member (e.g., a tension plate) extending in the module-layering direction outside the layered-module body.
In the solid polymer electrolyte type fuel cell, a reaction for decomposing hydrogen into hydrogen ion and electron occurs on the anode-side, and the hydrogen ion moves to the cathode side through the electrolytic membrane. A reaction for producing water from oxygen, hydrogen ion, and electron (the electrons produced in the anodes of adjacent ones of MEA's reach the cathode side through the separator, or the electrons produced in the anode of a cell at one end of the layered-module body reaches the cathode of a cell at the other end of the layered-module body through an external circuit) occurs on the cathode side.anode-side: H2→2H++2e−cathode side: 2H++2e−+(½)O2→H2O
In order to cool Joule heat and the heat of the reaction for producing water on the cathode side, a coolant flow channel through which a coolant (cooling water as a rule) flows is defined by adjacent ones of separators, so that the fuel cell is cooled.
Japanese Patent Application Laid open No. 2000-228207 discloses a metal separator according to an art related to the invention. This separator is formed by pressing a metal. A reactive gas and cooling water are caused to flow along front and back sides of the separator, respectively. FIG. 6 shows a separator 10 disclosed in the aforementioned publication. FIG. 7B is an enlarged view of an encircled region of the separator 10 which is indicated by A in FIG. 6, that is, a region around a joint portion between an opening portion 110 and gas flow channels 180. As shown in FIG. 7A, a gas flows from the opening portion 110 into the flow channels 180 via gas flow channel end portions 180a. Convex portions 190 separate the flow channels 180 from one another. FIG. 7B is a cross-sectional view of the gas flow channels taken along a line VB—VB in FIG. 7A. FIG. 8A is an enlarged view of an encircled region of the separator which is indicated by B in FIG. 6, that is, a region around a joint portion between an opening portion 150 and an end portion of a coolant flow channel 120. A coolant flows from the opening portion 150 into a channel 200 via a coolant flow channel inlet portion 200a. FIG. 8B is a cross-sectional view of the coolant flow channel taken along a line VIB—VIB in FIG. 8A. In the separator disclosed in the aforementioned publication, as shown in FIGS. 7A and 7B and FIGS. 8A and 8B, the width of the coolant flow channel is larger than the width of the gas flow channels. In addition to the separator disclosed in the aforementioned publication, other metal separators that have been actually manufactured are designed such that the width of a coolant flow channel is larger than the width of gas flow channels.
However, if a gas flow channel is defined by a front face of a separator that is designed to be pressed to define flow channels as in the case of a metal separator, the width and cross-sectional area of a coolant flow channel defined by a back face of the separator uniquely determined. In this case, if the width of the coolant flow channel is made larger than the width of the gas flow channel as in the case of the related art, the following problems are caused.                (1) A separator portion at a groove bottom portion of the coolant flow channel presses a diffusion layer and adversely affects the diffusibility of gas into a catalytic layer. Therefore, if the width of the coolant flow channel is increased, the areas of cells that can be effectively utilized to generate electricity are reduced. Since the amount of cooling water is increased and the metal has a high thermal conductivity, excessive cooling occurs, which tends to cause flooding in an oxidative gas downstream portion. Further, the amount of cooling water is increased and the thermal capacity of water is increased, whereby the cooling controllability may be deteriorated in some cases.        